Are all the prokaryotes unicellular, or can they be multicellular?
Are all the eukaryotes multicellular, or can they be unicellular also?

Your second question is easier to answer than your first. There are lots of unicellular eukaryotes, including amoebas, paramecium, yeast, and so on. As to whether there are multicellular prokaryotes, the standard answer is No, but there is a lot of evidence that some bacterial species can aggregate together and divide labor so that the "colony" is working more efficiently. This is characteristic of any traditional multicellular organism, but there's still a lot of resistance to the idea of calling these prokaryotes "multicellular."